FM06: Galactic Angular Momentum

The image represents a slice through a simulation volume, showing intergalactic gas colour-coded by temperature. The insets zoom to the stellar disks of a low-spin compact galaxy (left) and a high-spin grand-design spiral (right). (Virgo Consortium EAGLE simulation)

Angular momentum (AM) appeared as a key player when the modern theory of galaxy formation crystalised in the 1970s and early 80s. However, quantitative studies of galactic AM remained hampered by observational and theoretical obstacles. Advances by both camps over the last 10 years have produced major progress in galaxy evolution theory. The growing diversity of AM-related topics and the need to bring observers and theoreticians together is the driver behind Focus Meeting 6, beginning with a talk by distinguished Professor Emeritus P. J. E. “Jim” Peebles (Princeton University, New Jersey, US), a pioneer in tidal torque theory and father of the galaxy “spin parameter.”

Angular Momentum — A Catastrophe?

In the standard model of galaxy formation, dark halos grow through gravity from tiny primordial density fluctuations while acquiring AM through tidal torques. Galactic disks then condense at the halo centres by radiative dissipation of energy. The cooling baryons naturally exchange AM with their halos, but the mass-size relation of local star-forming spirals implies that, on average, the specific AM of these systems remains approximately conserved. Explaining this conservation has been a long-standing problem for theory: until early 2010, hydro-gravitational simulations systematically failed to reproduce disks as large and thin as those in normal late-type galaxies like the Milky Way. The simulated galaxies have been deficient in AM, making them too small and too bulgy — a problem so severe that it has become known as the “AM catastrophe.” Overcoming this catastrophe via stellar feedback has been one of the recent success stories of galaxy simulations. However, there is a new debate about the regulating mechanisms: Is outflowing gas torqued so that re-accreted gas has higher AM, is low-AM material preferentially removed from galaxies, or do the winds prevent loss of AM by making inflows smoother?

3D Cameras — A Challenge for the Hubble Sequence?

One lesson from the AM catastrophe is that AM is one of the most critical quantities for explaining galaxy morphologies, opening a new bridge between theory and observation. The recent rise of integral field spectroscopy (IFS) has enabled simultaneous measurements of the composition and Doppler velocity at every position in a 2D galaxy image, hence enabling a pixel-by-pixel integration of AM. Such measurements of AM in early-type galaxies have led to the surprising discovery that most seemingly featureless objects exhibit a rotational structure akin to that of normal spiral galaxies, thus possessing more AM than previously suspected. The fewer actual “slow-rotators” have as little as a tenth the AM at a fixed mass. AM thus offers a more fundamental, albeit harder to measure, classification of galaxy types than the classic Hubble sequence.

Similarly, analyses of spiral galaxies using spectroscopic and interferometric data reveal a tight relationship between the relative mass in the central stellar over-density and galactic AM, again suggesting that the Hubble morphological sequence might be replaced with a more physical classification by AM. The form of this new AM-based classification scheme remains nonetheless a source of great argument. Many recent hydro-gravitational simulations (e.g., Illustris, EAGLE, Horizon, Magneticum, MAGICC, CLUES, NIHAO) contribute to this discussion, as do most major kinematic observing programs. Prominent examples include optical IFS and IFS-like surveys (e.g., ATLAS3D, CALIFA, MaNGA, SLUGGS, PN.S, KROSS, SAMI), interferometric radio surveys (e.g., THINGS on the VLA) and other kinematic observations on modern instruments (e.g., KMOS, MUSE, SINFONI, Hector, ALMA, NOEMA).

Cosmic Evolution of Spin — A Clue to Galaxy Transformations?

The strong correlations between morphology and AM of local galaxies raises the question as to whether the cosmic evolution of morphologies is paralleled, or even driven, by the evolution of AM. Observationally, the Hubble Space Telescope‘s exquisite spatial resolution shows that star-forming galaxies at redshift z > 1 have very different structures from local grand-design spirals. Star-bursting early galaxies show a predominance of clumpy morphologies caused by supergiant (300 to 1000 pc) star-forming complexes. The physical origin of these clumpy morphologies, which always seem paralleled by high star formation rates and turbulence, is currently heavily debated. While high gas fractions could explain instabilities in spite of high dispersion, deep IFS studies (e.g., Keck-OSIRIS, Gemini-GMOS) in rare nearby clumpy disks suggest that low AM is the dominant driver of instabilities. This motivates the arguable conjecture that the cosmic evolution of AM indeed plays a major role in the morphological transformation of the star-forming population — a hypothesis to be debated in Vienna.

Answering Tomorrow’s Questions

Focus Meeting 6 will culminate with an exploration of pathways into the future. New facilities such as the James Webb Space Telescope and the Square Kilometre Array will revolutionise AM measurements at cosmological distances and large galactic radii. What will be the questions of tomorrow, and how can we tackle them using these instruments? What directions can simulations take to leverage these future observations? Join us at FM6 to contribute your ideas!

DANAIL OBRESCHKOW has done clean energy development and microgravity experimentation with the European Space Agency and now leads research on galaxy evolution and cosmology at the University of Western Australia node of the International Centre for Radio Astronomy Research.